A seatbelt for a motor vehicle typically has a seatbelt retractor that serves to retract the belt into a housing. The belt is wound upon a spool in the housing. When the belt is drawn or protracted, the spool winds a retraction spring, which later retracts the unused portion of the belt onto the spool.
In a crash the seatbelt retractor has a lock that limits the extension of the seatbelt from the housing. The lock may be actuated by an inertial sensor, which responds to changes in vehicle speed that occur in a crash. When a large deceleration is detected, the inertial sensor triggers the lock of the seatbelt retractor to secure the seatbelt in place or prevent further extraction of the seatbelt.
In a locked condition a conventional seatbelt system restrains the vehicle occupant from moving forward during a crash. Although the seatbelt has some give, the restraining force on the vehicle occupant can be significant. To reduce this force manufacturers may use an energy absorption mechanism, such as a torsion bar, to absorb energy from the forward movement of the vehicle occupant in a controlled manner. Generally, the spool is mounted on the torsion bar, which is mechanically linked to the spool. During a crash the torsion bar twists and deforms as the seatbelt is protracted. The deformation of the torsion bar absorbs energy from the seatbelt such that the vehicle occupant is stopped more gradually, rather than suddenly, during the crash.
The weight of the vehicle occupant can affect the rate at which the vehicle occupant is slowed by the restraining force of the seatbelt and torsion bar. Heavier vehicle occupants require a greater restraining force than lighter vehicle occupants. Therefore, it is desirable to use a higher rate of energy absorption for a heavyweight vehicle occupant than for a lightweight vehicle occupant.
Recently manufacturers began producing seatbelt retractors that absorb energy at different rates to accommodate differently weighing vehicle occupants. For example, when a small person is seated in the vehicle, the seatbelt retractor is set at a low rate of energy absorption such that the lighter weighing vehicle occupant is restrained with less restraining force than a heavier vehicle occupant. On the other hand, for a heavier vehicle occupant, a higher energy absorption rate is used to slow the heavier vehicle occupant with greater restraining force. A middleweight vehicle occupant may require a combination of restraining force rates during a crash. In this way a vehicle occupant receives a restraining force that better accommodates his weight.
In some situations a vehicle experiences successive crashes. It is desirable to continue to absorb energy at the same high rate for the heavyweight vehicle occupant in a second crash. However, for a lightweight vehicle occupant, it is preferable to absorb energy from the seatbelt spool initially at a low rate for the first crash, then at the higher rate for the second crash. Moreover, for a middleweight vehicle occupant, it is desirable to absorb energy at a high rate and then a low rate for the initial crash. For the second crash, a high rate of energy absorption is preferred. Conventional seatbelt retractors do not have such a feature. A need therefore exists for a multilevel energy absorbing retractor that solves the foregoing problem.